The viral vector core which will be responsible for construction of recombinant adeno-associated (AAV) and lentiviral (human immunodeficiency virus-based) vectors expressing transgenes of interest for in vivo gene transfer as needed in Projects. The efficiency, safety and level of transgene expressing will be ascertained. In addition, the Core will carry out research projects to explore and modify vector systems for achieving long-term expression of transgenes at physiologically significant but non-toxic levels in vivo. The core will also make tetracycline-regulatable recombinant AAV and lentiviral vectors and high tittered viruses for the in vivo gene transfer in the brain. In this system low concentrations of tetracycline, that have no toxic effects on cells, down-regulate transgene expression in a dose-dependent manner and completely abolish expression at higher doses. The system is reversible, as withdrawal of the effector molecule will restart transgene expression. The regulatable components of the tetracycline system have been incorporated into AAV and lentiviral vectors. These recombinant regulatable viruses are functional in vivo. To manipulate gene sequences in vivo, Cre-LoxP system have been explored. AAV and lentiviral vectors carrying bacteriophage P1 Cre-recombinase have been constructed for use in Projects. Tetracycline regulatable AAV and Lentiviral-Cre have been made in order to down-regulate Cre expression when needed. In another approach, to eliminate the toxicity of over-expression of Cre-recombinase in the brain, a new self-deleting lentiviral vector that is itself subject to Cre-mediated excision has been constructed and the recombinant virus has been tested for its action in the loxP "floxed" mice. Expression for Cre for only the time needed for combination eliminates Cre related toxicity. Core research will enable us to find the best vector system for in vivo gene delivery in the brain.